1. Field of the Invention
The present invention relates to a process for producing chain structured corpuscular calcium carbonate. More particularly, it relates to a process for producing chain structured corpuscular calcium carbonate having an average diameter of 0.01 to 0.1.mu. as primary particles and an average aspect ratio of 5 to 50.
2. Description of the Prior Art
Heretofore, the production of fine calcium carbonate by carbonation of calcium hydroxide has been carried out by introducing carbon dioxide into an aqueous suspension of calcium hydroxide.
In the conventional process for producing fine calcium carbonate, when carbon dioxide is continuously introduced into the aqueous suspension of calcium hydroxide, the viscosity of the suspension is gradually increased depending upon the progress of the carbonation. When 30 to 50% of the carbonation is performed, the viscosity of the suspension reaches to the maximum to cause viscous colloidal slurry by the formation of high molecular basic calcium carbonate. When the carbonation is further continued the viscosity is decreased to result a suspension having low viscosity as the original suspension. When the carbonation is further continued to the end point of the carbonation reaching the pH of the suspension to 7 to 8, the resulting suspension is filtered and the resulting mass is dried and pulverized.
The resulting fine calcium carbonate has been used as fillers for reinforcing rubber or plastics etc. It has been known that the reinforcing effect is higher depending upon reducing the diameter of the particles of fine calcium carbonate. However, the filler such as a special fine calcium carbonate having cubic shape has high mutual cohesive force between particles whereby the particles are distributed as aggregates in the kneading of the particles to the rubber or the plastics. It is difficult to disperse the particles as the primary particles.
Thus, the fine filler such as special fine calcium carbonate having cubic shape could not be uniformly dispersed into the rubber or the plastics, it has not been attained the expected reinforcing effect for the rubber or the plastics, disadvantageously.
On the other hand, chain structured fine calcium carbonate has the configuration developing to longitudinal direction by bonding primary particles in chain whereby the phenomenon of aggregation as it is found for the primary particles can be prevented whereby the dispersing property to the rubber or the plastics can be advantageously improved.
Moreover, when the chain structured corpuscular calcium carbonate is dispersed into the rubber or the plastics, the reinforcing effect for the rubber or the plastics could advantageously be improved because of the resulting steric structure. From these viewpoints, it has been desired to develop the production of chain structured corpuscular calcium carbonate having high aspect ratio.
For producing chain structured fine calcium carbonate, the following process has been known; the carbonation is carried out by introducing carbon dioxide gas into the aqueous suspension of calcium hydroxide and a water soluble sulfate, a zinc salt or a magnesium salt is added as an additive when the carbonation is performed to give the viscous colloidal suspension. However, only the aforementioned special additives have been effective and it has not been clear whether sulfates except sodium, potassium, lithium, aluminum, ammonium and ferrous sulfates could be effective or not. The additives which could be used have been limited.
Moreover, the aspect ratio of the chain structured fine calcium carbonate has been up to only about several and it has been impossible to obtain the chain structured corpuscular calcium carbonate having an aspect ratio of such as several tens. Furthermore, in order to complete the chain formation it has been needed to keep the resulting suspension in ageing for 1 to 7 days after the carbonation. These disadvantages have been found.
The inventors have studied to overcome the disadvantages and to attain the demands and the invention has been attained.